This invention relates to a novel additive for aqueous drilling fluids and a method for drilling wells in subsurface formations utilizing drilling fluids containing the additive. More particularly, the additive functions as a lubricant to substantially reduce torque in rotating drill stems.
Drilling fluids, or drilling muds as they are sometimes called, are slurries of clay solids used in the drilling of wells in the earth, particularly for the purpose of recovering hydrocarbons and other fluid materials. Drilling fluids have a number of functions, the most important of which are: lubricating the drilling tool and drill pipe which carries the tool, removing formation cuttings from the well, counterbalancing formation pressures to prevent the inflow of gas, oil or water from permeable rocks which may be encountered at various levels as drilling continues, and holding the cuttings in suspension in the event of a shutdown in the drilling and pumping of the drilling fluid.
An ideal drilling fluid is a thixotropic fluid, which is a fluid whose viscosity decreases as the degree of agitation or shear rate increases. Such fluids are desirable to reduce torque on a rotating drill string as well as support and remove formation cuttings from the well. Frequently, these two purposes are difficult to achieve at the same time. Consequently, certain materials are normally added to aqueous drilling muds for lubrication to decrease torque on the drill string.
U.S. Pat. No. 3,410,797 describes the lubricant use of a stannous dithiophosphate and a dibutyltin didecyl dithiophosphate in aqueous drilling muds. Phosphosulfurized olefins for drilling fluid lubricants are also disclosed in U.S. Pat. No. 3,810,836.
Examples of lubricants now used in aqueous drilling fluids include such proprietary materials as Torq-Trim II, a trademarked material sold by NL Baroid; Magcolube, a trademarked lubricant sold by Magcobar; Lubri-Sal, a trademarked additive sold by Milchem Inc.; and SWS, a trademarked lubricant sold by IMCO Services. These prior art additives may be used for lubrication in certain types of aqueous drilling fluids, most notably aqueous lignosulfonate muds. However, in other types of muds, they may be completely inadequate. Other lubrication additives include surfactants which create foaming problems and precipitation problems in hard waters. Polymer beads are sometimes employed to enhance the rotation of the drill bit but are difficult to work with primarily because their large size creates severe separation problems in the recycling of drilling fluids.
Special purpose drilling fluids are used in certain geographical areas where the well must penetrate formations known as heaving or sloughing shales. Aqueous drilling fluids containing hydroxides and other water soluble alkali metal and alkali earth metal salts have been frequently employed for drilling the type of heaving shales encountered in the Gulf Coast region of the United States. These lime based systems cannot be lubricated with the most common additives because the high pH and soluble calcium present in lime base systems render most drilling fluid lubricants ineffective. Fatty acid ester lubricants are hydrolized to fatty acids which precipitate out with calcium.
The additional fact that many Gulf Coast wells are highly deviated increases the need for lubricants to be used with the lime based systems frequently employed with unstable Gulf Coast shales. In deviated wells, greater lubrication is needed because the drill string is more likely to be lying against the side of the borehole. A deviated well also increases the likelihood of shale on the sides of the borehole becoming unstable, which increases the need to use lime based mud systems.
A wide variety of lubrication additives are commercially available for use with high pH lime drilling fluid systems. Only a few of these offer satisfactory torque reductions and most of those that provide substantial lubrication adversely affect the drilling fluid system.
When added to a lime:Mor-Rex system in 2% by volume quantities and tested according to Ex. 1, the trademarked product AZ-32 sold by Petroleum Associates of Lafayette, Inc. offered insignificant torque reductions of 3-4%. Similar results were found with the trademarked additives Lubrisal, Shale Heal and Torque Trim II, sold by Milchem, Gill Industries and NL Baroid, respectively. The trademarked additives G.M.L., ReDou Torque, and SDL-711, sold by Frontier Research, Oil-Base Germany and Superior Chemicals, respectively, yielded modest torque reductions of about 15-23%.
Magcolube, a trademarked product sold by Magcobar, Inc. yielded 22% reduction in torque but caused the drilling fluid to bead up and grease out the barite, an intolerable problem. Finally, E. P. Mud Lube, a trademarked product of NL Baroid caused foaming problems and also caused a greasing problem with the barite.
Accordingly, there is a substantial need for additives for use in aqueous drilling fluids, particularly hydroxide based mud systems, to lubricate the drill string and reduce torque loads.